Field of the Invention
The invention relates to a method of operating a gas and steam turbine plant, in which heat contained in an expanded working medium from a gas turbine, which is operated by a fuel, is used to generate steam for a steam turbine, and a reducing agent is introduced by a metering system, with a supply of an adjustable quantity of carrier air, into the working medium, for the catalytic cleaning of the working medium. The invention also relates to a plant operating in accordance with the method.
In a gas and steam turbine plant, the heat contained in the expanded working medium from the gas turbine is used for generating steam for the steam turbine. The heat transfer takes place in a waste-heat steam generator which is installed downstream of the gas turbine and in which heating surfaces are disposed in the form of tubes or tube bundles. The heating surfaces are in turn connected into a water/steam circuit of the steam turbine. The water/steam circuit usually includes a plurality of pressure stages, for example two. Each pressure stage usually has a preheater heating surface (economizer), an evaporator heating surface and a superheater heating surface.
The selective catalytic reduction method, the so-called SCR method, can be employed to decrease the amount of oxides of nitrogen in the exhaust gases of the gas turbine. In the SCR method, oxides of nitrogen (NO.sub.x) are reduced to nitrogen (N.sub.2) and water (H.sub.2 O) with the aid of a reducing agent and a catalytic converter.
In a gas and steam turbine plant constructed for an SCR method, a catalytic converter is generally disposed in the waste-heat steam generator. The catalytic converter is used to initiate and/or maintain a reaction between the reducing agent introduced into the exhaust gas and the oxides of nitrogen.
As is known from U.S. Pat. No. 4,473,536, the reducing agent required for the method can be injected with air generated by an air compressor or fan, as a carrier flow, into the exhaust gas from the gas turbine, that is flowing through the waste-heat steam generator. It is possible to vary the quantity of reducing agent to be injected by controlling the quantity of air as a function of the exhaust gas mass flow from the gas turbine.
As a rule, however, the emission of oxides of nitrogen from the gas turbine is higher in the case of oil operation than in the case of gas operation. In order to keep within the legally specified limiting values, the quantity of reducing agent to be injected for the oil operation of the gas turbine is therefore greater than that for gas operation. However, because of an undesirable cooling of the exhaust gas caused by the injection of the air/reducing agent mixture, the efficiency of the gas and steam turbine plant in that configuration can be impaired to a different extent, depending on the operating condition.